1. Field of the Invention
The present invention relates to a receiver/transmitter circuit, and more specifically to a receiver/transmitter circuit which reduces noise.
2. Description of the Related Art
A conventional receiver/transmitter circuit has an antenna terminal, a transmitter circuit for transmitting a transmission signal to the antenna, a receiver circuit for amplifying a reception signal from the antenna terminal, an antenna switch for selectively transmitting the transmission signal from the transmitter circuit to the antenna terminal and the reception signal from the antenna terminal to the receiver circuit, and a phase locked loop circuit which oscillates for frequency conversion at the transmitter circuit and the receiver circuit. The transmitter circuit includes a power amplifier (PA), and the receiver circuit includes a low noise amplifier (LNA) which amplifies the reception signal. The antenna switch includes a switch which connects the antenna terminal to the power amplifier and a switch which connects the antenna terminal to the low noise amplifier. The conventional receiver/transmitter circuit is disclosed in Japanese Patent Laid-Open No. 2002-72956.
However, the conventional receiver/transmitter circuit can radiate noise when switching from a receiving node to a transmitting node. There are two major causes of this problem. First, rapid switching of the antenna switch itself radiates noise via the antenna terminal. Second, a load change at an oscillator of the phase locked loop during switching of the antenna switch can cause the frequency at the oscillator to change, thus generating noise in the transmission signal.